Spring Specification -- Fatigue 1

[drawoh]

I am examining somebody's custom torsion spring and I want to make suggestions about their drawing. What I want is for the drawing to call up spring rate and maxiumum rotation. This is inspectable for us, as opposed to verifying the specific alloy and heat treatment (which they did not call up).

If I were concerned about metal fatigue, is there a crude rule of thumb that lets me specify an extension in excess of required that would allow for fatigue? For example, if my spring must exert 0.05N.m torque at 90[°], can I specify 0.1N.m torque at 180[°], and assume that anything that passes is safe?



--
JHG

 

[desertfox]

Hi drawoh

From my time designing torsion springs and a rule of thumb regarding fatigue we ensured that the tensile stress at the maximum deflection was no greater than 40% of the allowable stress.
So you need to calculate the stress in the spring at maximum deflection and compare it with the allowable for the specified material and wire size. Failing that it might be a good idea to talk to some spring manufacturer’s

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein